The advisory was issued Friday and continues through Monday. It includes the river and its tributaries upstream of the District of Columbia.
The National Weather Service is predicting heavy rain that could lead to a dramatic rise in water levels. At Point of Rocks in Frederick County, the weather service forecasts a 10-foot rise in the river level from Saturday night through Monday morning.
State regulators say high, rushing water carries debris that can easily destroy a boat or knock a swimmer or wading angler unconscious.